In the drying of paper and other materials in web form, cylindrical dryer rolls heated internally with steam are in common use. Steam is admitted to the interior of the rolls through roll journals equipped with rotary steam joints, and a mixture of steam, noncondensible gases, and condensate is drained from the interior by means of syphon pipes that pass through the journals. Control of drainage from the rolls is a difficult problem that frequently results in loss of drying capacity, loss of drying control, non-uniform drying, waste of steam, waste of cooling water for condensing waste steam, high maintenance cost, and high capital cost for equipment. The object of this invention is an improved method for controlling flow of drainage such that most of the problems are avoided.
FIG. 1 shows a typical arrangement for supplying steam and draining condensate and blow-through steam from a dryer roll. Although that arrangement of steam supply and syphon equipment is most typical, there are some variations. On very wide paper machines, the steam may enter through a rotary joint on one journal, and the syphon pipe may drain the dryer through a second rotary joint on the other journal. Another variation employs a stationary syphon pipe in which the syphon is held stationary as the roll rotates. The object of stationary syphons is to avoid the effects of centrifugal force on the fluid in the radial portion of the syphon. The problem with stationary syphons is that they cannot be mounted very close to the dryer shell without risking frequent breakage, and the rim of condensate therefore tends to be thicker in normal operation. Because of this problem, stationary syphons are much in the minority as applied to paper machines.
Condensate is formed within paper machine dryer rolls as steam is condensed on their interior surfaces, particularly when paper is being dried. At the high web speeds (1000 to 3600 feet per minute) in current practice, the condensate is pressed by centrifugal force against the inside surface of the roll shell to form a liquid rim within the dryer drum. At a web speed of 2500 feet per minute, for example, the centrifugal force acting on the condensate in a five foot diameter roll is over ten times the force of gravity. The liquid rim is not stagnant but oscillates with respect to the surface under the influence of gravity force as the roll rotates. In spite of this motion, the liquid rim interferes with heat transfer from the steam to the drying paper, and it has further been linked to non-uniform heat transfer in respect to edges of the drying web as compared to center.
When drainage of the liquid condensate, along with some steam, fails to occur on a continuous basis, the thickness of the liquid rim builds up to a point where the water cascades and ultimately collapses into a deep, agitated pond in the rotating roll. Thus when drainage fails the dryer becomes less and less effective until it contributes little to drying. Not only is drying capacity lost, but the heavy load of water causes breakage of syphons, severe loads on roll bearings, and high and unstable loads on the roll driving equipment. A primary requirement of the drainage method is therefore to maintain the thickness of the liquid rim as small as possible by adequately draining the dryer drums.
Air and other noncondensible gases also cause problems. All commercially generated steam contains a small fraction of such gases that must be purged continuously from any vessel in which the steam is condensed. If such gases are allowed to accumulate, they reduce the partial pressure and temperature of the steam. They further tend to concentrate locally near the surface of condensation and seriously impede heat transfer. When such gases are present they are not necessarily uniformly distributed in the steam space in a vessel and may cause great differences in heat transfer from one point to another on the condensing surface.
In the present state of the art the need to maintain the thinnest possible rim of condensate and to continuously purge noncondensible gases is recognized. Accordingly, dryer syphons are mounted as close as possible to the inside shell surface of the dryer roll, and a substantial amount of steam blows out of the roll through the syphon, entraining the condensate as well as purging out noncondensible gases. The condensation of part of the steam entering a dryer roll results in an increase in the concentration of noncondensible gases. Consequently, the blow-through steam contains a higher fraction of noncondensibles, but the fraction is usually very small because the incoming fraction is so small.
After the noncondensible gases have been purged out of the roll, they remain as a minor contaminant in otherwise valuable steam. The blow-through steam and noncondensible gases from all of the dryer rolls in a paper machine cannot be simply thrown away without great waste of heat energy. The efficient utilization of this contaminated blow-through steam is a primary objective of all steam control and dryer drainage systems.
When drainage occurs on a continuous basis from a dryer with rotating syphon, the pressure differential between dryer inputs and outputs necessary to drain the dryer depends on a composite of four primary pressure drop factors. These factors are:
1. friction and dynamic losses of essentially dry steam flowing from the steam inlet manifold to the interior of the dryer;
2. friction and dynamic losses of the two phase (liquid-gas) mixture flowing through the syphon to the drain manifold;
3. pressure loss in consequence of centrifugal force acting on the liquid portion of the fluid in the radial part of the rotating syphon pipe; and
4. pressure recovery in consequence of gravity force acting on the liquid portion of the fluid in the external piping draining downward from the dryer to the drain manifold.
Because each of the four differential pressure factors varies in a different manner as conditions change, the net differential pressure required to maintain drainage tends to be a complex function and varies substantially with conditions of operation. For example, machine speed primarily affects centrifugal force in the syphon and has little effect on friction losses. Steam pressure strongly affects friction losses and centrifugal force, and it also governs the rate of condensing. Under normal drying load, the rate at which paper is dried and the associated rate of condensing inside the dryer depend on the condensing temperature of the steam which is a function of steam pressure, i.e., an increase in steam pressure normally increases drying rate.
In order to demonstrate differential pressure effects, I have prepared FIG. 5, showing typical differential performance curves for a dryer in a large high speed paper machine. The dryer is equipped with a rotary syphon and is operating under normal drying load. The ordinate of the graph is the difference in pressure betweeen steam supply and drainage manifolds, which is called differential pressure. The abscissa is the amount of blow-through steam (steam that accompanies the condensate out through the syphon pipe) expressed as a percentage of condensing rate. There are two sets of three curves for three steam pressures, one set for a web speed of 3500 feet per minute (fpm) and one for 2500 fpm. The condensing rate is approximately constant at any given steam pressure, whatever the machine speed, and is highest at the highest pressure.
The curves of FIG. 5 are an extension of published research in which the nature of friction losses and centrifugal pressure losses with two phase flow in rotating syphons was described. I have extended this work to include inlet steam friction losses, external friction losses in two phase flow, and pressure recovery due to drop in level of two phase flow. More important, my curves fairly accurately predict the actual differential pressures that would occur at each steam pressure because I have also developed a method to accurately predict the actual condensing rate in the subject dryer, whatever its location in the drying process. Of special importance to the invention is the fact that the condensing rate is approximately proportional to the square root of the density of the steam in the dryer. The density of steam of course increases with pressure.
The curves of FIG. 5 clearly demonstrate the effects of centrifugal force. The difference between high speed and low speed sets of curves is primarily centrifugal force effect. The upward hook at the left ends of the curves is also a centrifugal force effect. At small levels of blow-through steam the radial portion of the rotating syphon pipe contains a greater proportion of liquid water in the liquid-gas mixture. Since only the water fraction of the mixture has significant mass, an increase in this fraction results in greater centrifugal force.
When stationary syphons are in use, the centrifugal force factor is not part of the differential pressure, and dryer speed does not affect the differential performance curves. If the curves for stationary syphons were to be plotted on FIG. 5, they would fall only slightly below the curves for 2500 fpm at high blow-through rates and would all approach roughly 1 pound per square inch (psi) at 21/2% blow-through, at which point they would nearly converge.
If dryer drainage stops for some time, it is necessary to use very high differential pressure to overcome centrifugal force acting on water alone in rotary syphons. The differential pressure needed to overcome the centrifugal force of water alone is about 10 psi at 2500 fpm and about 20 psi at 3500 fpm. Since it is often difficult to secure such high differential pressures on an operating machine, it is extremely important that drainage be maintained continuous on all dryers in a high speed machine.
In prior art practice with a group of dryers connected to inlet and outlet manifolds, the machine operator selects a differential pressure that he believes workable and sets the appropriate differential control instrument to maintain the selected differential pressure. Once set, the instrument is seldom reset unless some fairly obvious trouble develops. For example, with reference to FIG. 5, the differential setting could be as low as 6 psi for normal operation at 20 psi steam pressure and 2500 fpm machine speed. Upon increasing production by raising steam pressure to 50 psi and machine speed to 3500 fpm the dryers would stop draining and fill with water because 6 psi differential is not adequate for drainage at the new condition. The operator would in this case set the differential pressure between 8 psi and 12 psi by trial and error methods. Thus, with the prior art differential control the operator is blind as to whether or not the dryers are draining and is forced in most cases to set the differential pressure control much higher than necessary to make sure the dryers do drain. There is no way for him to measure or judge when differential pressure is excessive or insufficient. Even when some dryers stop draining, the operator may have no more than an indication that paper drying has been reduced but be unable to pinpoint which dryers or which section of dryers is at fault. This is a common occurrence on paper machines.
For operation according to the conditions shown in FIG. 5, the semipermanent differential pressure setting would ordinarily be about 9 psi and the blow-through rate would be about 27% at the highest pressure and speed. With conventional differential control, the 9 psi pressure would be maintained at all times, even when operating at 20 psi at 2500 fpm, to avoid the problems which occasionally result when a lower differential pressure is used. In this case the blow-through rate would be about 34%, which is unnecessary and expensive. Not uncommonly, system specifications require operation at an input pressure of 0 psi (gauge pressure), in which case the blow-through rises to 39% at 2500 fpm.
In order to operate with adequate drainage at 0 psi steam pressure, a group of dryers must discharge a mixture of steam and condensate to a drainage manifold maintained at a substantial negative pressure or vacuum (9 psi vacuum in the above example) to maintain the differential pressure required to drain the dryers. Ordinarily such low drainage pressures could only be obtained by discharging the drain manifold directly to a vacuum system. A vacuum system usually consists of a condenser, vacuum pump, and condensate collection tank with condensate pump. The first few dryers in a paper machine are normally operated at an input steam pressure of 0 psi or less, and their blow-through steam and condensate are discharged directly to a vacuum system.
In the case of a main group of dryers, it is impractical to discharge all of the blow-through steam to a vacuum system because the resulting large waste of steam cannot be tolerated. On the other hand recompression and recirculation of the blow-through steam from the vacuum has been impractical because the specific volume of the blow-through steam under vacuum is so large that an extremely large thermocompressor, consuming an overwhelming amount of motive steam, was required to recompress it. Furthermore, an oversized thermocompressor is incompatible with dryer drainage requirements at higher steam pressures. It is also difficult to perceive how steam pressures could be maintained so low without some direct connection to a vacuum system.
Also of importance is the fact that two phase flow in dryer piping is highly erosive. In the above cases the reduced pressures and increased blow-through result in very large increases in the velocity of two phase flow through syphons and external piping. Erosion of dryer drainage piping is a common problem in practice.
What happens to dryer drainage upon loss in drying load, as during web breaks on paper machines, is also important. I have prepared the graph, FIG. 6, to illustrate the effect of load loss on differential performance. In this graph, I have used the gravimetric flow rate rather than percentage of blow-through steam as the abscissa. The upper curve corresponds to normal condensing load at the indicated conditions and is identical to the corresponding curve in FIG. 5 except for the scale of the abscissa. The lower curve is based on the same conditions, except the condensing rate is reduced to 12 percent of the normal rate.
With the prior art differential control the amount of blow-through steam increases as the condensing rate falls. In the case of a web break, if differential pressure were maintained at 8 psi, the blow-through rate would increase from about 580 pounds per hour (lb./hr.) under normal load to about 960 lb./hr. when condensing rate falls to 12% of normal. This large excess of blow-through steam during web break conditions is extremely difficult to handle. The usual result is that the major part is dumped into the condenser, and with most of the controlled groups of dryers dumping into the condenser the condenser becomes pressurized and control is lost. The resulting overpressurizing of some dryers and loss of drainage in others complicates rethreading the wet paper web on the dryers and re-establishing control of steam pressure and dryer drainage.
A common way to avoid this problem is to greatly increase the size of the condenser and its cooling water system. This does solve the control problem. Moreover, condensers and cooling water systems are expensive, and much steam is wasted in prior art systems during web break conditions.
In most paper machines, groups of dryer rolls are connected to piping manifolds to simplify control. For example, a paper machine with 40 dryers might be divided into four groups of varying numbers of dryers, each group being controlled as a unit. All of the dryer rolls in a given group are connected to a common steam supply manifold and to a common dryer drainage manifold mounted below the elevation of the dryers.
FIGS. 2 and 3 illustrate two typical steam routing and pressure control systems of the prior art for one group of eight dryers. Most paper machines have several such groups in which the number of dryers may range from two to about thirty. The two typical systems differ primarily in that the cascade type system of FIG. 2 is dependent on further sections of dryers at lower steam pressure to consume blow-through steam. The thermocompressor system of FIG. 3, on the other hand, is independent of the other sections but requires a continuous bleed of steam to a condenser to provide continuous discharge of noncondensible gases. Although pneumatic controls are shown in FIGS. 2 and 3, controls with equivalent electronic signals are also in use and the references to pneumatic controls herein apply equally well to electronic controls.
In FIG. 2 steam is supplied from steam supply line 30 through control valve 31 to inlet manifold 32 that supplies steam to the dryer rolls 20. The dryer rolls 20 drain through their syphons to manifold 33, and the mixture of blow-through steam and liquid condensate flows to the separator tank 34. The condensate is separated from the blow-through steam and returned to the boiler by means of pump 35 through control valve 36. The nearly dry blow-through steam leaves the top of the separator and flows through control valve means 37 and a check valve 29 to the next section of dryers. Should the next group of dryers be unable to absorb all of the blow-through steam, part of the flow will pass through control valve means 38 to a condenser type heat exchanger maintained at low pressure or vacuum. This latter portion of the steam is condensed to recover the condensate and to return it to the boiler. All of the latent heat of this latter steam is lost, and a substantial further cost is involved in providing cooling water to effect the condensation. It is therefore important to avoid costly loss of steam through valve 38 to the condenser.
In FIG. 2 the pressure transmitter 39 measures the steam pressure in manifold 32 and transmits a proportional pneumatic pressure signal to pressure control instrument 40. The controller 40 compares this signal to its set point pressure and transmits a pneumatic pressure signal to control valve 31 to decrease or increase steam pressure as required. The standard pneumatic signal has a pressure range of 3 to 15 psi. In the case of an air-to-open valve like valve 31 in FIG. 2, the valve begins to open at 3 psi and is wide open at 15 psi. The control signal continues to increase from 3 psi until the valve is sufficiently open to maintain the steam pressure set in the controller.
Differential pressure transmitter 41 measures the difference in pressure between inlet and outlet manifolds 32, 33 and transmits a signal that is a measure of the differential pressure to controller 42 which in turn transmits appropriate signals to the control valves 37 and 38. These valves are "split ranged" so that valve 37 starts to open at 3 psi and is wide open at 9 psi air signal. Valve 38 starts to open at 9 psi and is wide open at 15 psi. Usually the steam system is designed so that in normal operation an air signal of less than 9 psi is ample for control because valve 37 will pass all of the blow-through steam necessary to maintain differential pressure and none will be wasted through valve 38. A drawback to this cascade system is that the next section of dryers must be maintained at significantly lower steam pressure and must be able to absorb all of the blow-through steam if waste is to be avoided.
Another problem with the cascade method of dryer drainage shown in FIG. 2 is that the differential pressures required between sections are cumulative, so that the first section must always be operated at rather high pressure. Typically, dryers running at 2500 fpm surface speed require 6 to 8 psi differential pressure between inlet and outlet manifolds and a further 2 to 3 psi differential from outlet manifold through the separator and piping to the inlet of the next group of dryers. Thus in spite of the fact that the third group may discharge into a substantial vacuum (7 to 10 psi vacuum is common), the minimum workable steam pressure in the first section may be greater than 20 psi. This is much too high for good operating control of most paper machine dryers, and the problem becomes much worse with the higher speeds that are now common. Accordingly, the plain cascade system as above described is rapidly becoming obsolete except for older and slower machines.
The thermocompressor system of FIG. 3 is similar to FIG. 2 except that the blow-through steam is recirculated rather than being passed to another group of dryers. In order to do this the lower pressure blow-through steam must be recompressed to the inlet manifold pressure. This is commonly done by a steam jet compressor 43 that uses the potential energy of high pressure steam to do the work of compression. Both recompressed blow-through steam and spent motive steam are discharged into the inlet manifold. The amount of motive steam required and the size of the thermocompressor required depend on the amount of compression work to be done. Compression work increases with greater differential pressure, with greater recirculation flow, and with lower pressure steam because the specific volume of the steam to be compressed is larger. A significant amount of steam must be bled out to the condenser through bleed valve 45 in order to prevent the accumulation of noncondensible gases in this otherwise closed system.
In FIG. 3, pressure controller 40 normally controls valve 31, which opens over a signal range of 9 to 15 psi, with an air signal greater than 9 psi to maintain steam pressure in the inlet manifold. Differential controller 42 normally controls the motive steam flow in thermocompressor 43, which opens over a signal range of 3 to 9 psi, with an air signal less than 9 psi to maintain differential pressure as required by the control set point. The output signal of both controllers enters a signal selector relay 44 which selects the lower signal and transmits it to the thermocompressor 43. Thus if drying steam demand drops, as when no paper is being dried, the air signal from pressure controller 40 drops, initially closing valve 31 and eventually dropping low enough to take over control of the thermocompressor 43 and limit the supply of motive steam to the dryers as well. Meantime the reduced flow of motive steam reduces differential pressure, causing the air signal from differential controller 42 to increase until valve 38 opens to waste steam to the condenser. In this way both pressure and differential pressure control are maintained at all times. Although the thermocompressor system isolates each group of dryers, which simplifies the operation and control of paper machines, it consumes high pressure steam from line 28 that would otherwise be used for power generation. In practice this is quite wasteful as well, steam being wasted to the condenser when differential pressures are set high or when inlet pressure is low under which conditions the thermocompressor is frequently not large enough to do all of the recompression work. Even more important the waste of steam through bleed valve 45 becomes quite excessive in practice.
Although a continuous bleed of roughly 5 percent of the steam supplied is sufficient to purge noncondensibles, the working bleed rate is commonly in excess of 10 percent. A thermocompressor system is usually intended to operate over a wide range of controlled steam pressures, and it is essential that the bleed rate be adequate at the lowest pressure. The adjustable bleed valve is accordingly set manually for what is estimated to be adequate for low pressure operation. In practice this initial setting tends to be substantially more than 5 percent of the input steam to make sure that noncondensible gases will be purged. However, normal operation is usually at medium to high steam pressures and the loss of steam through the bleed valve becomes several times greater than that necessary at lowest pressure. The result is an unnecessarily waste of steam and high energy cost for drying paper.
The turndown control ratio of the thermocompressor system of FIG. 3 is also restricted on high speed machines. The greater dryer pressure differential required for high speed operation creates a need for more compression work to return the recycled steam to working pressure. At low steam pressures, the necessary compression work is more than even a large thermocompressor can do efficiently because the amount of high pressure motive steam required for compression becomes greater than the amount of steam that can be condensed in the dryers. Not uncommonly, the lowest controllable steam input pressure for a group of dryers having a recycle thermocompressor is 15 psi or higher. Also, thermocompressors are very expensive and there is a strong tendency to undersize them in practice.
The conventional differential pressure control method illustrated in FIGS. 2 and 3 is unreliable and wasteful because it does not respond correctly to the requirements of dryer drainage. Even at normal load conditions of operation, it causes excessive rates of blow-through steam at excessive differential pressures. These normal excesses result in larger thermocompressors consuming unnecessarily large amounts of high pressure motive steam. At other than normal operating conditions, as during paper breaks (no drying load) or during abnormally low steam pressure for drying at low rates, large amounts of steam are wasted to the condenser and frequently control is lost. Flooded dryers and breakage or harm to syphons are common in the industry.
An improvement over the conventional differential control was proposed and patented by U.S. Pat. No. 2,992,493, issued to Fishwick on July 18, 1961. Fishwick proposed to control dryer drainage in cascade type systems by controlling the flow rate of blow-through steam rather than the differential pressure between the steam intake and exhaust of individual dryers.
The numerous advantages anticipated by Fishwick never materialized. Fishwick expected that the amount of blow-through steam and the corresponding differential pressures would be reduced, and the result would be improved design of dryer drainage systems, steam savings, and lower operating pressures. However, Fishwick's control method in itself did not reduce the amount of blow-through steam or differential pressure needed by any group of dryers under specific operating conditions. Consequently, most of the problems of the cascade system, particularly the lack of control range, remain unabated.
The principle of blow-through control taught by Fishwick has been applied only to thermocompressor systems in Yankee dryers, in which a single dryer replaces the group of dryers shown here and the gravimetric rate of flow of blow-through steam from the separator 34 is measured by a flow meter and controlled by controller 42. Yankee dryers are normally operated with high steam pressures and with high rates of bleed steam, and in consequence derive little benefit from blow-through control.